This invention relates generally to a planetary gear assembly. More particularly it relates to a helical planetary gear assembly which may be used in the transfer case of a modern four-wheel drive vehicle.
When a four-wheel drive vehicle is operated off-highway, it requires a reduction unit having a gear ratio lower than that required for on-highway use. This is in order to provide reduced vehicle speed, improved gradeability, and ease of traverse over adverse terrain. A lower ratio also is required to keep engine speed sufficiently high for adequate fan cooling under slow running conditions.
The typical reduction unit incorporates either a layshaft gear set or a planetary gear set. As compared with a conventional layshaft gear set, a planetary gear set is preferable in that it affords lighter weight, compactness, reduced size, and improved efficiency and durability. These features are becoming increasingly desirable as the cost of energy and equipment increases.
Further, it is accepted practice today to provide a vehicle with an engine of reduced size. In such a case, an associated reduction unit would be used extensively, even at relatively high speeds. Where a reduction unit using spur gears, which are inherently noisy, heretofore was acceptable, it is now preferable to use helical gears in order to provide quieter operation.
For these and other reasons, there has developed in the art a need for a reduction unit incorporating a planetary gear set with helical gear teeth. However, in a helical planetary gear set the sun and ring gear helix angles generate thrust loads which are difficult to contain. This is particularly so where a gear set rotates as a unit in the direct drive mode. It is important that these thrust loads be contained. It also is important that the gears not be loaded when a planetary gear set is operated for long periods in the direct drive mode so as to avoid wearing flats on the gear teeth contacts, and thus reduce noise and vibration when operation is in the reduction drive mode. Finally, it is important to have a construction which permits the gears to seek centers for the best sharing of torque loads, thereby further reducing noise and improving durability.